(i) Field of the Invention
The invention relates to the classes of objects known as sailboards and inflatable water craft. In particular, it is scaled to meet the needs of children, ages eight to twelve years. It is also a multipurpose water craft, as it can be used with or without it's sail, for different purposes. The invention is demountable, portable and storable in a confined space.
(ii) Description of the Prior Art
Children currently use adult sailboards to learn to sailboard. These boards are difficult for them to handle in the water, during transportation, and for storage, being both heavy (more than 40 lb/18.2 kg), and long (over 11 feet/365 cm). A beginner board requires high floatation (over 200 liters), and current boards achieve this through material and length. The described invention reduces the weight and the length, while maintaining the required floatation, by the use of inflation. In addition, the use of inflation in the design adds the attributes of the board being collapsible into manageable pieces, and storable in confined spaces.
The following are quantitative considerations for the design of a sailboard using a stiffened inflatable structure.
Three main considerations govern the design of sailboards--buoyancy, weight and stiffness.
With respect to buoyancy, sailboard has to have buoyancy to support the total operating weight (weight of the user plus weight of the board with all its gear). Buoyancy is obtained as a function of the board's total volume V, as follows: ##EQU1## where L=board length;
T=board thickness PA1 B=board breadth; PA1 factor=depending on shape. PA1 W=V.times.D.sub.av PA1 D.sub.av =function of various D.sub.comp PA1 W=board weight; PA1 D.sub.av =average board density PA1 D.sub.comp =board component density PA1 D.sub.S =skin density; PA1 D.sub.c =core density. PA1 D.sub.i =inflatable wall density PA1 L.sub.p =length of platform PA1 T.sub.p =wall thickness of platform PA1 D.sub.p =density of platform PA1 A.sub.T =cross-section of stiffening tubes PA1 D.sub.T =density of stiffening tubes PA1 a) locally, to resist footprint pressures and mast and daggerboard reactions PA1 b) generally, to resist hydrodynamic forces and retain streamline shape.
For an adult beginner's board, total operating weight is usually around 125 Kg. Typical board volume is therefore about 250 liters.
For a child's board (8 to 12 years old child) total operating weight can be reduced to about 85 Kg and the board's volume can be kept to 170 liters.
With respect to board weight, the lighter the board, the easier it is to handle, both on land and in the water, and the less is its contribution to total operative weight and required buoyancy. Weight is a function of volume and density, as follows:
where
In conventional board design (foam core, reinforced skin) average density can be approximated as: ##EQU2## where T.sub.S =skin thickness;
Typically D.sub.ao can attain values as low as 1/10 water density, and corresponding board weight is EQU W=250 1/10=25 Kg.
In a stiffened inflatable board design, as proposed in this invention, average density can be approximated as: ##EQU3## where: T.sub.i =inflatable wall thickness
Because of the design choices offered in such a design (low weight of inflatable, limited size of platform, tailored stiffening structure), D.sub.av can be further reduced to values as low as 1/20 water density. In the case of the child's board, this results in a boardweight of: EQU W=170 1/20=8.5 Kg.
With respect to stiffness, the board is subjected to forces which require structural strength and stiffness:
Local forces are accommodated by structural design of the specific components, such as mast foot, daggerboard well, etc., both in conventional designs and in the proposed inflatable stiffened design. In the latter, these features are part of the platform design, made more efficient by the use of structural foam plastic.
General board stiffness can be characterized in a simplified analysis by considering longitudinal (bow to stern) bending stiffness. A convenient, non-dimensional indicator can be defined as: ##EQU4##
Typical conventional boards achieve S values of 500 N/cm, or a stiffness to weight ratio of 200.